Electrical impedance transformer for use at very high frequencies



1956 w. J. VAN DE LINDT 2,768,356

ELECTRICAL IMPEDANCE TRANSFORMER FOR USE AT VERY HIGH FREQUENCIES Filed May 9, 1951 INVENTOR WILLEM JACOBUS 'VAN DE LINDT AGENT United States Patent ELECTRICAL llVIPEDANCE TRANSFORMER FOR 7 USE AT VERY HIGH FREQUENCIES Willem Jacobus van de Lindt, Eindhoven, Netherlands,

assiguor to Hartford National Bank and Trust Company, Hartford, Conn., as trustee The invention relates to electrical impedance transformers for usev at very high frequencies and to circuitarrangements comprising. such transformers.

A known impedance transformer comprises two waveguides, each of which is closed at one end by an adjustable piston. The wave guides are coupled with one an other through an aperture of adjustable size in a common wall portion. With the use of this transformer an im pedance of substantially any value can be transformed into substantially any other impedance value. However, this device has certain disadvantages. In the first place it is not suitable for high power, since at the aperture of adjustable size high field strengths occur, giving rise to break-down. Moreover, it requires. to be calibrated experimentally. Particularly for measuring purposes there is a need for an impedance transformer, in which the relation between the adjustment and the complex transformation ratio can be accurately calculated in advance in a simple manner, so that calibration is not required.

The object of the invention is to provide an improved electrical impedance transformer. 7

According to the invention, an electrical impedance transformer for use at very high frequencies, comprising two wave guides, each of which is closed at one end by an adjustable piston, is characterized in that in at least two spatially separated areas provision is made of coupling elements between the wave guides in a manner such that a travelling wave moving in the direction of the piston and the coupling elements in. one wave guide exclusively results in travelling waves of equal amplitudes moving in the two wave guides from the coupling elements to the pistons.

It should be noted that such a coupling, which is termed directional coupling is known per se. In this case the coupling elements are, consequently, not adjustable. Therefore the requirement that the field strength should be minimum anywhere can be beter observed when designing the transformer. Thus the impedance transformer according to the invention is suitable for high power.

The phase and the amplitude of the transformation ratio vary in a readily calculable manner with the position of the pistons, so that experimental calibration may be dispensed with.

In a preferred embodiment of the impedance transformer according to the invention the two pistons are coupled by two control-members, of which one exclusively determine the difference between and the other the sum of the spacings between the pistons and the same coupling element. As will be described hereinafter the first control-member is adapted to adjust the amplitude, the second the phase of the voltage reflection coefiicient, the two adjustments being independent of one another. It may be assumed to be known that the trans formation ratio can be calculated in a simple manner from this reflection coeflicient.

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In order that the invention may be more clearly understood and readily carried into effect, it will now be described more fully with reference to the accompanying drawing, in which:

Fig. 1 shows on embodiment of an electrical impedance transformer according to the invention and Figs. 2, 3 and 4 show vector diagrams, with reference to Which the operation of the transformer shown in Fig. 1 will be explained.

The impedance transformer shown in Fig. 1 comprises wave guides 1 and 2, which are closed on therighthand side by pistons 3 and 4, respectively. The lefthand ends 5 and 6 of these wave guides serve as the input and the output of the impedance transformer, respectively. The wave guides 1 and 2' are intercoupl'ed by a number of coupling elements, which are in this case constituted by apertures 7 in the common side wall 8 of the wave guides 1 and 2.

Onthe right-hand side of the wave guides provision is made of means for adjusting the pistons 3 and 4, com prising a frame 9, in which a plunger part 10 is slidably arranged. The part 10 is: adjustable by means of a screw 11. The part 10 is provided with a set screw 12, a part 13 of which. has a left-handed thread and apart 14 of which, otherwise exactly similar to the part 13., has a right-handed thread. Nuts 15 and 16, co-operating with the parts 13 and 14, respectively, are connected by way of rods 17 and 18, respectively, with the pistons 3 and 4', respectively. If the screw 11 is turned, the two pistons 3 and 4 will move in the same direction through equal distances, whereas if the screw 12 is turned the pistons 3 and 4' will be shifted through equal distances in opposite directions. The three planes 19, 20 and 21, shown in broken lines are at right angles to the longitudinal axis of the wave guides and pass through the right-hand end of a coupling aperture 7 adjacent the pistons and the planes of the operative surfaces of the pistons 3 and 4, respectively. The distance between the planes 19 and 20 is a1 and that between the planes 19 and 21 is a2. The value of the expression ar-l-az varies only with the position of the screw 11 and the value of the expresion aza1 only with the position of the screw 12.

The operation of the device will now be explained with reference to the vector diagrams shown in Figs. 2 to 4. It is assumed that the end 6 of the wave guide 2 (Fig. 1) is closed by means 22 so as to be free from reflection.

Considering a travelling wave, which enters the wave guide 1 at the end 5, the amplitude of the field strength being V, it appears that owing to the effect of the directional coupling, at the right-hand side of the plane 19 a travelling wave moving to the right occurs in the two wave conductors. For the case under consideration, in which the directional coupling is arranged in a manner such that these waves have equal amplitudes, it follows from the directional coupling theory that these waves have a phase difference of relative to one another and a phase diiference of 45 relative to the iniital wave in the absence of a coupling between the wave guides.

This condition is illustrated in Fig. 2, V1 and V2 denoting the amplitude and the phase of the waves in the wave conductors 1 and 2 at the instant when the plane 20 is reached. V1 lags by a phase angle of 90 relative to V2. From the reciprocity theorem it follows that, if the direction of movement of these waves is reversed, the wave V will emerge from the end 5. This condition is illustrated in Fig. 3. Since the wall 8 is a plane of symmetry, the wave V will emerge from the aperture 6, if the phases of V1 and V2 are interchanged.

Fig. 4 illustrates the efiect of the pistons 3 and 4 on the waves V1 and V2 of Fig. 2, moving to the right. In the area of the plane 20, the wave V1 will be reflected by the pistons 3. In this case the phase is shifted by 180. Thus the Wave V1 is produced moving to the left. If the operative surface of the piston 4 were also located in the plane 20, the wave V2 would also be reflected at the plane 20, so that the Wave V2, travelling to the left would be produced. In this case V1 would be 90 out of phase with V2. With the pistons in the positions shown, the wave reflected by the piston 4, having arrived back at the plane 20, will, however, have experienced a phase shift 8, owing to its having twice covered distance aza1. Here where A=the wave length of the wave travelling in the wave guides.

The waves V1 and V2, both moving to the left in the wave conductors 1 and 2, respectively, may be resolved in the manner shown into two pairs (AB and CD) of waves, each one of a pair being equal in amplitude to the other and out of phase therewith by 90. The pair AB produces a wave emerging from the aperture 6, which is completely absorbed here by the means 22. The pair CD produces a wave emerging from the aperture 5, the amplitude of which is equal to 5 V sin 2 Since 5 varies with the expression az-ai, only the amplitude of the wave emerging from the aperture 5 is affected by turning the screw 12. Consequently, this amplitude may be varied substantially between and V. Upon turning the screw 11, the pistons 3 and 4 are shifted by equal distances, so that equal phase shifts of the waves V1 and V2 will occur. The resultant R of the pair of waves CD, emerging from the aperture 5, will be subjected, in this case to the same phase shift, which phase shift will be equal to where K is a constant determined by the length of the wave guide 1. If desired, the screws 11 and 12 may be constructed as micrometer screws. It will be obvious that the phase variations in and the amplitude of the wave emerging from the aperture can be calculated from the displacements of the pistons 3 and 4.

Thus the ratio between the incoming and the outgoing waves can be adjusted at will, this is to say that on the input side 5 of the transformer substantially any desired impedance can be realized if the transformer is closed on the output side so as to be free from reflection. It appears from theory that a device having this property has a transformation ratio, which is adjustable at will, for substantially any output impedance.

What I claim is:

An impedance transformer comprising input and output waveguides having end portions thereof positioned mutually parallel and having a common side wall, a directive coupler between said waveguides comprising apertures in said common wall, first and second pistons respectively disposed in said end portions to provide reflective terminations for said waveguides, and means for sliding said pistons in and parallel to the axes of said waveguides, said means comprising a frame extending from said end portions and having an elongated opening, the axis of said elongated opening being parallel to the axes of said waveguides, a plunger slidably positioned in said opening, an elongated screw member rotatably held by said plunger, the axis of said screw member being parallel to the axes of said waveguides, said screw member being rotatable about its own axis with respect to said plunger and comprising a left-hand thread and a right-hand thread respectively centered on said lastnamed axis, nuts respectively engaging said threads, connective members respectively connecting said nuts with said pistons, means for permitting manual rotation of said screw member to cause said pistons to slide in opposite directions in said waveguides, and means for permitting manual sliding of said plunger in said opening to cause said pistons to slide together in the same direction in said waveguides.

References Cited in the file of this patent UNITED STATES PATENTS 2,428,485 Carter Oct. 7, 1947 2,486,818 Bowman Nov. 1, 1949 2,508,573 Hulstede May 23, 1950 2,564,030 Purcell Aug. 14, 1951 2,568,090 Riblet Sept. 18, 1951 2,593,120 Dicke Apr. 15, 1952 OTHER REFERENCES Publication I-Directive Couplers in Wave Guides by Surdin; published in the Journal of the Institution of Electrical Engineers, vol. 93, part IIIA, No. 2, 1946, page 66. Copy in Patent Oifice Library. 

